I am learning javascript and i came across a doubt. Why is the value of "this" undefined in the first example , but prints out correctly in the second.
example 1:
var myNamespace = {
myObject: {
sayHello: function() {
console.log( "name is " + this.myName );
},
myName: "john"
}
};
var hello = myNamespace.myObject.sayHello;
hello(); // "name is undefined"
example 2:
var myNamespace = {
myObject: {
sayHello: function() {
console.log( "Hi! My name is " + this.myName );
},
myName: "Rebecca"
}
};
var obj = myNamespace.myObject;
obj.sayHello();//"Hi! My name is Rebecca"
Why does the value of "this" changes within the function. What concept am i missing?
The this keyword refers to the current object in a method or constructor. The most common use of the this keyword is to eliminate the confusion between class attributes and parameters with the same name (because a class attribute is shadowed by a method or constructor parameter).
By default, "this" is a reference to the object on which a particular function is called (in JavaScript all functions are bound to an object). We can, however, use call() and apply() to change this binding at runtime.
In JavaScript, the this keyword refers to an object. Which object depends on how this is being invoked (used or called). The this keyword refers to different objects depending on how it is used: In an object method, this refers to the object.
A variable that has not been assigned a value is of type undefined . A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned .
First case you are just getting the reference of the function to the vairable hello
, and invoking it from global context (window in browsers, global in node), So this
becomes what invoked the function except for (bound functions). You can always set the context explicitly using function.call or set the context explicitly to the function using Ecma5 function.bind
hello.call(myNamespace.myObject); //now you are setting the context explicitly during the function call.
or just bind it while getting the function reference.
var hello = myNamespace.myObject.sayHello.bind(myNamespace.myObject); //Now no matter where you call it from `this` will point to the context of myObject
Second case you are invoking it from the object itself so this
points to the object.
In the first case, the implicit this
object is the global scope. Because there is no myName
in the global scope, you get undefined.
If you want a free function with the proper this
, use bind
:
var hello = myNamespace.myObject.sayHello.bind(myNamespace.myObject);
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